Hi, what regulations should I use to assess a 100 year old industrial building which was originally a print works which has had a change of use and is now a craft shop with a small warehouse.  I assume it'll be offices and shops? The travel distances are different for each guide and will conflict with what is currently in place. Thanks.

Possibly you can use both - Offices & Shops for that part of the building and Factories & Warehouses for the other, assuming the layout is such that one distinct part of the building is the warehousing and the other the shop.

You aren't always forced to use one guide for the whole of a building if it has different usage groups, just the appropriate one for the use, occupancy and risks of the distinct areas. If they are so overlapping and intermingled it may be different - without seeing it I can't be definitive.

Thanks Anthony.

In this part of the world you would need both planning permission and building control approval for such change of use. The latter would likely insist that TBE (NI equivalent of ADB), or BS 9999 or fire engineer justification was employed. The exercise in seeking statutory approval would steer the FRA to large extent. 
A change of use application here is normally tested as if the building was new, especially in safety critical aspects.